An undercarriage may comprise firstly a main undercarriage close to the center of gravity in order to support a major fraction of the weight of the aircraft, and secondly an auxiliary undercarriage arranged at the front or the rear of the aircraft to contribute to steering the aircraft when taxiing on the ground. In this context, an undercarriage arranged at the front of the aircraft is referred to as a “nose” undercarriage and an undercarriage arranged at the rear of the aircraft is referred to as a “tail” undercarriage.
An auxiliary undercarriage is thus steerable, unlike a main undercarriage.
In addition, when landing gear is configured in the so-called “tricycle” configuration, it has two main undercarriages and one auxiliary undercarriage.
Under such circumstances, the undercarriage with a rocker lever may be used as an auxiliary undercarriage for an aircraft, in particular for a helicopter.
When used as an auxiliary undercarriage, it is necessary to mount the twin wheels on a swivel so as to make steering on the ground possible.
FIG. 1 is a diagrammatic view of a rocker lever undercarriage, the undercarriage 1 having a rocker lever 2, a swivel 3, and a twin wheels 4 shown in an ideal static position on the ground. The twin wheels 4 comprise an assembly of two wheels (only one wheel R is visible in FIG. 1) that are connected together by a shaft referred to herein as a “wheel axle”. The wheels of the twin wheels are placed on either side of the midplane of the swivel 3 and of the rocker lever 2.
The swivel 3 then presents a first portion 3b that is fastened to the rocker lever 2 and a second portion 3c that extends along a swivel axis A1 extending along the middle of the swivel 3, the second portion 3c being fastened to the twin wheels 4. Under such circumstances, the second portion 3c is free to swivel about said swivel axis A1 relative to the first portion 3b. 
In this position, the swivel axis A1 of the swivel 3 is substantially perpendicular to the ground S. The wheels R of the twin wheels 4 bear against the ground S in symmetrical manner.
However, since an aircraft fitted with such a rocker lever undercarriage presents weights in use that vary approximately over a range of one to two between a minimum weight and a maximum utilization weight, the rocker lever undercarriage is subjected to different forces in use. Thus, in use, it can happen that the static deflection of the undercarriage varies and that the swivel axis A1 of the swivel is no longer perpendicular to the ground.
FIG. 2 shows an example of a position for the rocker lever undercarriage in which the swivel axis is no longer perpendicular to the ground, with this occurring for example when the aircraft is light in weight or else when it has its center of gravity offset longitudinally a long way towards the tail.
The elements 2, 3, and 4 of the rocker lever undercarriage 1 remain unchanged compared with FIG. 1. However, the swivel axis A2 of the swivel 3, in this example position, is no longer perpendicular to the ground S.
To a certain extent, such an inclination for the swivel 3 is not a particular impediment, providing the twin wheels 4 can perform their function. However, in the event of turning or towing with a lateral component, the effect of the swivel 3 being inclined means that the twin wheels R of the twin wheels 4 are no longer symmetrical and it can happen that one of the wheels is lifted off the ground.
FIG. 3 shows an example of one of the wheels of a rocker lever undercarriage being lifted off the ground. Because of the inclination of the swivel axis A2 of the swivel 3 relative to the vertical, the first wheel R1 of the twin wheels 4 is no longer in contact with the ground S, so the twin wheels 4 rests on the ground S solely via its second wheel R2.
This phenomenon presents five negative effects:
increasing the load on the wheel that remains on the ground, thereby reducing its lifetime or indeed requiring it to be reinforced, thus requiring additional material for constructing it, thereby increasing its size and its weight and simultaneously increasing its manufacturing cost;
generating an additional twisting moment in the elements of the undercarriage because of the asymmetry of the reaction at the two wheels of the twin wheels;
generating a return movement about a vertical axis that gives the aircraft a small amount of under-steer for small angles and a small amount of over-steer for large angles;
when turning, since the wheel carrying the load is the inner wheel, this reduces the size of the stability triangle of the aircraft when turning; and
the visual appearance is unsightly and mechanically unsound.
To mitigate those drawbacks, proposals have been made in a first solution to use a single wheel, since the phenomenon only occurs when using a twin wheels. That solution is sometimes also used for a tail undercarriage.
The use of a single wheel serves to solve only three of the above-mentioned drawbacks, i.e. load distribution, twisting moment, and reduced stability in turning. There nevertheless remain the above-mentioned drawback concerning return moments, even if it is slightly reduced, and also the drawback relating to appearance. Although those two remaining drawbacks may be acceptable for a tail undercarriage, e.g. a tail wheel, this is not true of a nose undercarriage.
Furthermore, the use of a single wheel requires a wheel to be provided that is of a size larger than the size of the two wheels of a twin wheels. Using such a wheel in an undercarriage that is retractable thus presents the drawback of requiring a larger wheel bay.
A second solution that has been proposed is to use an active mechanical element enabling the two wheels of the twin wheels to remain in contact with the ground.
In a similar context, document CA-A-523 199 describes an undercarriage enabling an aircraft to be controlled on the ground by steering the twin wheels about a vertical axis.
That document proposes compensating either the angle of inclination of the wheel axle of the twin wheels or incorporating a mechanical system for generating different speeds of rotation for the two wheels of the twin wheels.
That therefore requires the addition of a complicated active compensation system, thereby increasing the cost of such an undercarriage as well as its size and its weight.
Furthermore, that document does not deal with the problem caused by the swivel of the undercarriage being inclined relative to the vertical.